A typical electronic ballast for the HID lamp is known to include a DC—DC converter providing an increased DC output, and an inverter which converts the DC output into an AC power for driving the lamp. The DC—DC converter includes a switching element which is driven to repetitively interrupt an input DC voltage to regulate the resulting DC output. Also included in the ballast is a controller which monitors the DC output and varies the duty of the switching element of the DC—DC converter based on the monitored DC output in order to regulate the DC output in a feedback manner, thereby generating a pre-starting voltage required for the lamp to be ignited, in addition to a maintaining voltage required to keep operating the lamp once started.
In order to turn on the lamp, the DC—DC converter is firstly controlled to increase the output DC voltage to a pre-starting voltage which is sufficiently high to make the lamp ready for being ignited for initiating the discharge of the lamp. In order to successfully start the lamp, the lamp has to be kept at the pre-starting voltage for a certain time period until the lamp is ignited. Generally, the pre-starting voltage is defined as a high limit voltage that the DC—DC converter is designed to keep generating until the lamp is ignited, i.e., while there is no-load connected to the converter. However, the time required for the DC—DC converter to increase its output DC voltage from zero to the pre-starting voltage will depends upon the input DC voltage available from a DC voltage source. Accordingly, in case where the available input DC voltage is not expected to be constant, the pre-starting voltage is reached at different times. That is, when the available input DC voltage lowers, the pre-starting voltage can be reached after a longer time, thereby correspondingly delay the starting of the discharge lamp. Since the input DC voltage lowering is likely in actual application environments, such delay has to be avoided in order to satisfy a demand of starting the lamp constantly at a uniform response and without being conscious of the input DC voltage variation.